The present invention relates to a surface inspection apparatus for inspecting an aspherical body of an optical system, such as a convex lens, concave lens, etc., for surface profile condition by the hologram interference process.
Conventional inspection apparatuses of this type are known which use an interferometer e.g., a Twyman Green interferometer. FIG. 1 shows one such prior art apparatus. In this inspection apparatus, a laser beam emitted from a laser generator 1 is diverged and collimated by a magnifying optical system 2, and then projected onto a translucent mirror 3, where it is divided into an object light 4a and a reference light 4b. The object light 4a is applied to an aspherical surface 6 as a subject surface to be inspected through an irradiation lens 5. The object light 4a reflected by the nonspherical surface 6 is passed through the irradiation lens 5 and the translucent mirror 3, and then projected onto a computer hologram 7. Out of the diffracted rays of the object light 4a which has passed through the hologram 7, zeroth-order diffracted rays or unmodulated transmitted rays 4a' are converged by a converging lens 8, passed through a spatial frequency filter 9 with an orifice, and projected onto a screen 10 to form an image. The reference light 4b is reflected by a reflector 11, and then further reflected by the translucent mirror 3 to be passed through the computer hologram 7. Out of the diffracted rays of the reference light 4b which has passed through the hologram 7, positive first-order diffracted rays 4b' are converged by the converging lens 8, passed through the spatial frequency filter 9, and projected onto the screen 10 to form an image thereon. Other diffracted rays of the object and reference lights passed through the hologram 7 are filtered out by the filter 9.
In the inspection apparatus constructed in this manner, the unmodulated transmitted rays 4a' of the object light 4a which have passed through the computer hologram 7 interfere with the positive first-order diffracted rays 4b' of the reference light 4b selected by the filter 9 at the outlet side thereof, and interference fringes are projected on the screen 10. If the unmodulated transmitted rays 4a' and the positive first-order diffracted rays 4b' are equal in phase (i.e., if the subject surface 6 is an ideal surface free from distortion), then the interference fringes will be straight by adjusting the optical axis and the focal position of each lens. If the subject surface 6 is distorted, the object light 4a, after being projected onto the subject surface 6, is reflected in a slightly different direction from its course of incidence, and interferes with the positive first-order diffracted rays 4b' of the reference light 4b. As a result curved portions appear in the interference fringes. Thus, the surface condition or the finishing accuracy of the subject surface 6 can be judged from the bend of the interference fringes.
In such a prior art inspection apparatus, however, the intensity of the diffracted rays 4b' of the reference light 4b is lower than that of the transmitted rays 4a' of the object light 4a, causing an imbalance between the rays. Thus, the interference fringes obtained are blurred, and it is hard to accomplish a high-accuracy inspection.
In the conventional inspection apparatus, moreover, the positive first-order diffracted rays of the reference light transmitted through the hologram are selected by the filter 9. As shown in FIG. 2, however, the space frequency distribution of the reference light transmitted through the hologram is such that positive first- and second-order diffracted rays are located very close to each other. In order to extract only the positive first-order diffracted rays with high accuracy, it is necessary to make the spatial frequency f of a carrier wave sufficiently high. If the maximum frequency bandwidth of the transmitted rays 4a' of the object light is .DELTA.f, the space frequency f of the carrier wave in the inspection apparatus must satisfy a relation f&gt;3.DELTA.f. If the spatial frequency of the carrier wave is made higher, however, the number of curves described on the computer hologram increases correspondingly. Thus, the manufacture of the computer hologram becomes elaborate, resulting in an increase in the manufacturing cost of the inspection apparatus. Also, it is hard to manufacture the computer hologram with high accuracy, and the inspection accuracy is lowered.